The present invention relates to an optical head for focusing an optical beam for recording/reproducing information or recording, reproducing and erasing information on an information recording medium such as an optical disk and, more particularly, to a focus detecting system for detecting the displacement of the focus of an optical head.
Recently, various systems for recording information on an information recording medium, such as an optical disk reproducing or erasing the recorded information, have been developed and proposed. In these systems, a beam spot corresponding to a beam waist of an optical beam converged by an objective lens at recording, reproducing and erasing time is required to be formed on an optical disk. Thus, an optical head has a focus detecting system for detecting the focused state of the lens. For example, a focus detecting system which is called a so-called "knife edge method" was proposed as disclosed in U.S. patent application Ser. No. 399,873 filed on July 19, 1982 by the same assignee as this application, and an EPC application No. 82106508.3, filed on July 19, 1982 as a focus detecting system. In this focus detecting system which is called the knife edge method, a light shielding plate which shields part of an optical beam reflected from a light reflecting surface and permits passage of other components is arranged between the light reflecting surface of the optical disk, i.e., information recording surface and a photo-detector for detecting the focus. The other beam component is directed between a pair of photo sensitive regions of a photo-detector at the focusing time, and the level of the detected signals from a pair of photo sensitive regions is maintained substantially and equally. The other beam component is directed to one of the pair of photo sensitive regions of the photo-detector at nonfocusing time, and the level of the detected signals from the pair of photo sensitive regions is unbalanced. Therefore, whether the objective lens is focused or not can be judged according to the detected signals from the pair of photo sensitive regions.
In the focus detecting system, such as the so-called "knife edge method," it is required to accurately dispose the light shielding plate in an optical passage of the light beam, and it is necessary to pay careful attention in alignment at assembling time, and there arises a problem that the assembling work becomes complicated. Further, a space for disposing the light shielding plate is required, the space should be sufficiently large to simplify the assembling work, and the system itself cannot be accordingly reduced compact in size.